Many say it’s the oxygen getting into the wine that softens the tannins. I agree the tannins soften once the wine has been through the Vinturi but I don’t think it’s just because of the O2 getting in. I think it’s because of the CO2 getting out. It’s been well documented that young red wines bottled with relatively high levels of dissolved carbon dioxide (DCO2) can taste quite firm and tight. The higher the DCO2 the more pronounced and firm the tannins.